Here’s a link to the easiest method I’ve seen for flashing a sonoff.
by DrZzs
I’ll leave this description here for posterity, but the process has change. Refer to the new video above for the updated and improved process.
Sonoff switches are one of the best deals in Home Automation. A wifi chip with a relay that is powered by 110v for $5! And the BEST part is how flexible and capable they become when you flash them with TASMOTA firmware, which makes them easy to connect and control with Home Assistant.
via YouTube https://youtu.be/chyVjtYb0EA